A so-called classification tree has been widely used as a category determination algorithm applicable to general pattern recognition problems. Generally, a pattern recognition problem can be formulated as a “problem for determining a specific category to which a test pattern expressed as a given point in a feature space belongs when a learning pattern is given as a point set in the feature space”.
A variety of algorithms have been proposed for this pattern recognition problem through many years. Among them all, classification trees are the most popular algorithms each for deciding a boundary for classifying category regions on the feature space.
The present applicant has proposed a method of forming a hierarchical structure for each feature amount axis (also referred to as a dimension axis hereinafter) of a feature space and forming a classification tree on the basis of the hierarchical structure in Japanese Patent Application No. 9-187019 (U.S. application Ser. No. 09/112,448, issued as U.S. Pat. No. 6,560,359, issued May 6, 2003).